I have heard that from several sources as well, I heard that its because when they stand up and "box" opponents in play, I have heard its because of the shape of their head...

Originally their name was from their ancestors being the Bullenbeisser (Bull biter) due to the nature of their work or taking down large animals such as bulls, cows, bears. It is generally accepted that the smaller Bullenbeisser bred in Brabant(?), together with the British Bulldog, is the ancestor of the modern Boxer dog. Then after that I guess they became enormously popular in Germany and were bread extensively, I read somewhere that they obtained the name "Boxl" there and the name "Boxer" is just the misinterpretation of that original name....